Typical magnetic carrier for use in electrophotographic magnetic brush development along with toner is iron powder and ferrite particles having a resinous coating. The magnetic carrier is effective in triboelectrically charging the toner whereby the toner adheres to the carrier by an electrostatic force and then transferred to a photoconductor upon development.
Therefore, the magnetic carrier particles are required to have a sufficient triboelectric charge in a uniform manner to pick up the toner uniformly for subsequent deposition. The carrier particles should also be efficient in carrying the toner in the developing unit and be free flowing powder.
Further, the carrier particles function as one electrode in the developing zone for producing a uniform electric field. They are thus required to have a desired resistance for a particular type of copying machine within the range of from 10.sup.5 to 10.sup.12 .OMEGA. by changing the composition of magnetic particles on which a resinous coating is applied or changing the composition of the resinous coating on magnetic particles. It is desired that the electric resistance of the carrier particles do not lower under humid conditions.
Another requirement imposed on the carrier is durability in that the carrier can maintain and perform its function consistently in the developing unit.
However, the prior art resin coated carriers have several problems including a broad distribution of electric charge, low coating strength, a slow rise of triboelectric charging upon replenishment of toner, a change of electric charge with time, and the fusion of toner to carrier (known as toner spent). As a result, images become poor in quality aspects such as image reproduction and resolution. As copying operation is continued over several ten thousand sheets, many quality factors including image density, fog, reproduction and resolution will lower with time and carrier adhesion and toner scattering occur.